The present invention relates, in general, to the rehabilitation of chloride contaminated reinforced concrete structures and, more particularly, to a method for rehabilitating chloride contaminated reinforced concrete structures and a new class of materials to accomplish same.
Steel reinforcing rods contained within concrete are protected against corrosion by the alkalinity of the cement within the concrete. The cement contains alkali, alkali earth, metal oxides and hydroxides that typically result in concrete having a pH of between 12 and 14 depending upon the source of the cement and its age. In a highly alkaline environment, the steel reinforcing rods are passivated by the formation of a surface oxide film that protects the steel from corrosion. This protective oxide film is relatively stable at pH values greater than approximately 9.5 in a chloride free environment. The pH value required to stabilize the protective oxide film increases as the chloride content within the concrete increases. If this protective film is broken, corrosion can commence on the steel reinforcing rods. This may occur as a result of the ingress of sufficient chlorides into the concrete matrix to initiate corrosion. The chlorides may originate from the use of deicing salts, exposure to a marine environment, or through the use of a concrete admixture that contains chlorides. Alternatively, or in combination with the ingress of sufficient chlorides, carbonation of the concrete can occur. Carbonation, which is the reaction of CO and CO2 in the air with available alkali in the concrete, causes the pH of the concrete to decrease over time. Once the pH of the concrete is below 9.5, the protective oxide film starts to break down resulting in the commencement of corrosion of the steel reinforcing rods and the deterioration of the concrete structure. The deterioration of such structures has become a concern in the concrete industry. This concern has become so important that the issue of concrete durability has replaced the issue of concrete strength as the most pressing problem facing the concrete industry.
The objective of any type of concrete repair is for the repair to be relatively low in cost and durable in nature. In addition, variations in the repair should be limited and predictable over time and the repair should not deteriorate over time. Typically, there are two approaches to rehabilitate chloride contaminated reinforced concrete. One approach is to remove the damaged concrete and replace it with patch materials. Another approach is to utilize electrochemical means to minimize or eliminate future corrosion of the steel reinforcing rods within the concrete. Electrochemical chloride extraction typically involves the application of relatively high direct electrical currents to the concrete over a period of 10 to 50 days. The objective of this approach is to remove 20-50% of the chlorides from the concrete. Cathodic protection involves the passage of a small direct electrical current through the concrete. The objective of this latter approach is to reduce the rate of reinforcing rod corrosion to very low levels that are not of engineering significance.
To apply an electrical current to the concrete, an anode is attached to the concrete and a voltage is applied between the anode and the steel reinforcing rods causing a direct current to flow through the concrete. If the voltage originates from the natural difference in the potentials of the anode and the steel reinforcing rods, the foregoing system is known as a galvanic system. An alternate approach, known as the impressed current system, utilizes a rectifier to provide the voltage for the resulting direct current system. With the cathodic protection approach, it is generally assumed that the protective current must be continually provided to the steel reinforcing rods.
In existing steel reinforced concrete structures, the deterioration process of the reinforcing rods can reach different stages depending on the age of the structure, the exposure conditions, any cover provided for the concrete, and the overall quality of the structure. If a corrosion situation is possible or at the onset of corrosion, preventive measures may be initiated. In contrast, in severely corroded steel reinforced concrete structures, repairs to the structures must be performed. Some producers of rehabilitation materials have asserted that their materials can stop or delay the initiation of corrosion on the steel reinforcing rods within the concrete. Typically, such materials must be applied to the concrete surface. Many of these materials can be classified as migration corrosion inhibitors. Application of such materials to the concrete surface requires that the material penetrates through the concrete to reach the steel reinforcing rods and that the material be of sufficiently high concentration to reduce the corrosion rate of the rods. According to technical literature and information, different classes of materials have been considered to be corrosion inhibitors. Typically, such materials are organic amines (hydroxylamines, hydroxyalkylamines). As a result of the volatility of organic amines, these materials have the properties necessary to migrate into dry mortar or concrete. No information is available, however, regarding the ability of these materials to migrate into wet concrete.
In some instances, the possibility of using vanadium in corrosion inhibiting formulations has been addressed in publications. For example, in U.S. Pat. No. 6,048,413 (Park, et al.) a corrosion resistant duplex stainless steel having an austenite ferrite duplex phase matrix is disclosed. In this case, the stainless steel contains a lower content of nickel and exhibits a higher resistance to stress corrosion cracking and pitting in environments containing chloride ions. The stainless steel is comprised of 20-30% chromium, 3-9% nickel, 3-8% molybdenum, 0.20% or less carbon, 0.20-0.50% nitrogen and the balance of iron. This stainless steel may also include at least one element selected from the group of 1.5% or less titanium, 3% or less tungsten, and 2% or less vanadium.
As an example of vanadium being contained within a pigment, U.S. Pat. No. 5,037,478 (Okai, et al.) discloses a corrosion preventive pigment consisting essentially of a phosphorus compound (phosphorus pentoxide, orthophosphoric acid, condensed phosphoric acid, alkaline earth metal or transition metal phosphate, or alkaline earth metal or transition metal condensed phosphate) and a vanadium compound which generates a vanadate ion in the presence of water and oxygen (vanadium oxide, vanadyl compound, an alkaline earth metal or transition metal vanadate, a baked condensate of alkaline earth metal or transition metal vanadates, or a heterocondensate of alkaline earth metal or transition metal vanadates).
Vanadium has also been mentioned as an ingredient in corrosion inhibiting systems for gas conditioning solutions. For example, U.S. Pat. No. 4,372,873 (Nieh) discloses a corrosion inhibited composition consisting essentially of an aqueous alkanolamine, an anion containing vanadium in the stage of oxidation +4 or +5 having a concentration of at least 100 ppm, and an amine co-inhibitor having a concentration in the composition of at least 0.4% (by weight) selected from the group consisting of methyliminobispropylamine and lower alkyl, N-hydroxyalkyl substituted derivatives where lower alkyl has 1 to 4 carbon atoms.
Vanadium has also been mentioned as an ingredient in chromate-free coating mixtures and coatings formed from these mixtures that protect an underlying aluminum or aluminum alloy substrate from corrosion. The use of vanadium salts as components of the chromate-free coating mixtures is disclosed in U.S. Pat. No. 6,077,885 (Hager, et al.). The coating mixtures contain vanadate salts of alkali and alkali earth metals, such as sodium and calcium metavanadate. The corrosion inhibiting coatings disclosed in this patent not only provide protection for aluminum and its alloys against corrosion, but the coatings also have a xe2x80x9csite blockingxe2x80x9d, or xe2x80x9cbufferingxe2x80x9d action, in that the inhibitors of the coatings are, to some extent, mobile and migrate into damaged areas of the aluminum and/or alloy to protect the damaged area from corrosion. This mobility is the result of the solubility of the corrosion inhibitors in the polymer matrix of the coating.
In view of the possibility that the use of vanadium as an active ingredient in a steel repassivator for corroded steel reinforcing rods in chloride contaminated reinforced concrete structures might produce an improved repassivator, it has become desirable to investigate whether vanadium is a proper ingredient in such a repassivator and, if so, to develop a repassivator containing vanadium.
The present invention provides a new active steel repassivator for the rehabilitation of chloride contaminated reinforced concrete structures and a method to accomplish such rehabilitation. The new active steel repassivator is comprised of a continuous phase, a distributed phase, and/or an aqueous solution of an amine. The continuous phase, also known as the carrier phase, is selected from organic strong volatile bases (cyclohexylamine, dicyclohexylamine, guanidine), salts of weak volatile acids (benzoic esters, butyl cinnamates, nitrobenzoates, alkaline or earth alkaline metal beuzoates, or carboxylates), cycloamide hydroxysubstituted, and a non-ionic surfactant with either polyether or polyxydroxyl polyhydroxyl as a polar group, 75-99%, by weight. The distributed phase includes an anion containing vanadium in the +4 or +5 state of oxidation and having a concentration in the composition of 100-300 ppm. The aqueous solution of an amine co-inhibitor has a concentration in the composition of at least 25% by weight, selected from the group consisting of triethylenetetramine, ethylenediamine, propylenediamine, piperazine, N-amiothylpiperazine. By applying the aforementioned active steel repassivator directly to the surface of the chloride contaminated reinforced concrete, the active steel repassivator penetrates, migrates and contacts the surface of the corroded steel reinforcing rods within the concrete. Electrochemically, the active steel repassivator of the present invention shifts the corrosion potential of the steel reinforcing rods from the active corrosion state to the passive state. To keep the rehabilitated steel reinforcing rods in the passive state, a sealer or polymer coating can be applied to the surface of the reinforced concrete.